Sulfur Doping Induces Strong Ferromagnetic Ordering in Graphene: Effect of Concentration and Substitution Mechanism Jiri Tucek, Piotr Blonski, Zdeněk Sofer, Petr Simek, Martin Petr, Martin Pumera, Michal Otyepka, Radek Zboril* Advanced Materials 2016, 28(25), 5045-5053